News Release Number: STScI-2001-20

Hint of Planet-Sized Drifters Bewilders Hubble Scientists

June 27, 2001: Piercing the heart of a globular star cluster, NASA's Hubble Space Telescope uncovered tantalizing clues to what could be a strange and unexpected population of wandering, planet-sized objects. The orbiting observatory detected these bodies in the globular cluster M22 by the way their gravity bends the light from background stars, a phenomenon called microlensing. These microlensing events were unusually brief, indicating that the mass of the the intervening objects could be as little as 80 times that of Earth. Bodies this small have never been detected by microlensing observations.

Q & A: Understanding the Discovery

1.
What are these "mystery objects"?

Theoretically, these mystery objects might be planets that were gravitationally torn away from their parent stars in the cluster. (Normally, planets orbit stars, just like Earth circles the Sun.) However, astronomers estimate that the wandering celestial bodies make up as much as 10 percent of the cluster's bulk - too numerous to be "orphaned" planets. The objects could be brown dwarfs, "failed stars" that are 80 times more massive than Jupiter. They're called failed stars because they don't have enough hydrogen in their cores to shine as stars. Or, these small, dim bodies could be a new class of objects.

Astronomers are planning more Hubble observations of the center of this dense cluster of 10 million stars. They expect to detect another 10 to 25 microlensing events, enough to yield direct measurements of the true masses of the small bodies.

2.
How does microlensing work?

Microlensing is an indirect way of obtaining information on celestial objects, such as planets or brown dwarfs, which are too dim to see.

Here's how microlensing works: As an unseen body floats across the face of a background star, it acts like a powerful lens by gravitationally bending the starlight and thus creating two separate images of the faraway star. Even Hubble can't resolve these images, because the bending angle is about 100 times smaller than the telescope's angular resolution. But the object's gravity also amplifies the starlight, causing it to brighten as the body passes in front of the star.

Microlensing events are brief, unpredictable, and rare. Astronomers improve their chances of observing one by looking at many stars at once. That's why astronomers pointed Hubble at a dense cluster of 10 million stars. They figured that some of the stars had planets or other objects around them. Some of the objects eventually might pass in front of background stars, causing microlensing events. In this case, the background stars are in the galactic bulge of our Milky Way Galaxy. Only the "sharp eyes" of Hubble can penetrate the core of a dense cluster of stars and monitor stars well beyond that region.

The telescope monitored 83,000 stars every three days for nearly four months, looking for slight increases in starlight. A spike in brightness provided evidence that an unseen body in the jam-packed cluster was passing in front of a star. The increases in brightness were transitory. Six of them lasted less than 20 hours and one stretched for 18 days. Based on the duration of the eclipse and the amount of brightening of background starlight, astronomers estimated the masses of the unseen bodies. In the case of the six short microlensing events, the objects are much smaller than a normal star, perhaps 80 times Earth's mass. The 18-day microlensing event was caused by a star. A typical star like our Sun is about 300,000 times more massive than Earth and 1,000 times heftier than Jupiter, the solar system's most massive planet.